Recently, there is proposed a digital video tape recorder for compression-coding video data by using a compression coding technique such as MPEG2 and recording the compression-coded stream onto a magnetic tape.
In accordance with the MPEG2 standard, a bidirectionally predictive coding system is employed as the coding system. In this bidirectionally predictive coding system, three types of coding, that is, intra-frame coding, inter-frame forward predictive coding, and bidirectionally predictive coding, are carried out. Pictures of the respective types are referred to as I-picture (intra coded picture), P-picture (predictive coded picture), and B-picture (bidirectionally predictive coded picture).
In general, I-picture, obtained by intra-frame coding, is not a picture predictively coded from another picture but a picture singly coded within this picture. P-picture, obtained by inter-frame forward predictive coding, is a picture predictively coded from past I-picture or P-picture. B-picture, obtained by bidirectionally predictive coding, is a picture bidirectionally predicted from past I-picture or P-picture and future I-picture or P-picture.
Also, in accordance with the MPEG standard, it is prescribed that an I-picture should be periodically inserted to enable random access. The unit thereof is referred to as GOP (Group Of Pictures).
At broadcasting stations, editing processing for editing a plurality of original video materials recorded on magnetic tapes so as to produce a television program to be on the air is carried out. In the case where such editing processing is carried out, edit points such as IN-point and OUT-point are set on the frame basis, and the plurality of video materials are cut and connected on the basis of the set edit points.
However, P-picture and B-picture included in a coded stream in conformity to the MPEG standard are pictures predictively coded from a past picture or a future picture, and are closely interrelated with the past or future picture. Therefore, a problem arises such that the coded stream cannot be cut or connected at an arbitrary frame position on the stream.
For example, if two different coded streams are forcedly connected with each other at an edit point, the picture quality of a decoded image is considerably deteriorated because a predictive picture of a picture near the editing point differs from a picture at the time of coding. Also, in some cases, since the coding parameter becomes discontinuous at the edit point, decoding cannot be carried out.
In short, in the conventional digital video tape recorder for recording a coded stream which is compression-coded in accordance with the MPEG standard, it is difficult to edit a coded stream recorded on a magnetic tape on the frame basis (picture basis). In addition, in the conventional digital video tape recorder in accordance with the MPEG standard, it is difficult to edit a coded stream recorded on a magnetic tape without deteriorating the picture quality.